public IReadOnlyList<string> GetFallbackTools()

{
  // A fallback tool is the tool that will be activated when an editing tool
  // is deactivated; (when the Escape key press for example)

  // To register a tool as a fallback tool, add the following into your
  // add-in's config.daml file

  // <updateModule refID = "esri_editing_EditingModule">
  //   <menus>
  //     <updateMenu refID = "esri_editing_DefaultTools">
  //       <insertButton refID = "MyAddIn_MyCustomTool"/>
  //     </updateMenu>
  //   </menus>
  // </updateModule>

  var options = ApplicationOptions.EditingOptions;

  // get the list of fallback tools that are registered
  var fallbackTools = options.GetFallbackTools();
  // get the current fallback tool
  var fallbackTool = options.FallbackTool;

  // set the fallback tool
  // (MyAddIn_MyCustomTool must be added to the "esri_editing_DefaultTools" list)
  options.FallbackTool = "MyAddIn_MyCustomTool";
}